Method and apparatus for performing idle handoff in a multiple access communication system

ABSTRACT

In a mobile communication system having a first communication device for communicating with a plurality of second communication devices, a method for performing handoff within the mobile communication system includes transmitting from at least one of the second communication devices to the first communication device a handoff list of second communication devices to which the first communication device is permitted to handoff. A channel assignment message is transmitted from each of the second communications devices in the handoff list. The handoff list includes base stations controlled by a single controller that determines which of the second communication devices transmits the channel assignment message. The base stations on the handoff list have pilot signals and the energy levels of the pilot signals are measured by the first communication device. The measured energy levels are transmitted to the controller and compared by the controller to provide an energy level list of base stations according to the comparing. The energy level list is transmitted to the first communication device and the handoff is performed according to the energy level list.

CLAIM OF PRIORITY UNDER 35 U.S.C. §120

The present Application for Patent is a Continuation of patentapplication Ser. No. 09/626,758 entitled “Method and Apparatus forPerforming Idle Handoff in a Multiple Access Communication System” filedJul. 27, 2000, now allowed, which is a Continuation of patentapplication Ser. No. 09/168,191, filed Oct. 7, 1998, entitled “Methodand Apparatus for Performing Idle Handoff in a Multiple AccessCommunication System,” now issued as U.S. Pat. No. 6,178,164, whichclaims priority to Provisional Patent Application No. 60/061,626, filedOct. 9, 1997, and assigned to the assignee hereof and hereby expresslyincorporated by reference herein.

BACKGROUND

I. Field

The present invention relates to communication systems. Moreparticularly, the present invention relates to a novel and improvedmethod and apparatus for performing idle handoff in a multiple accesscommunication system. In addition, the present invention relates to animproved method for assigning a traffic channel in a multiple accesscommunication system. The invention also relates to a method forreducing the number of required handoffs occurring while a mobilestation is queued and waiting for a traffic channel in a multiple accesscommunications system.

II. Background

Typically, communications systems prohibit handoffs while a mobilestation is in a system access state. The system access state is thestate in which communications are initiated either by the mobile stationby means of transmissions over an access channel or by a base station bymeans of transmissions over a paging channel. In the exemplaryembodiment, messages are sent in accordance with a code divisionmultiple access (CDMA) communication format, which is disclosed indetail in U.S. Pat. No. 4,901,307 entitled “Spread Spectrum MultipleAccess Communication System Using Satellite or Terrestrial Repeaters”and U.S. Pat. No. 5,103,459 entitled “System and Method for GeneratingWaveforms in a CDMA Cellular Telephone System,” both of which areassigned to the assignee of the present invention and are incorporatedby reference herein. The use of paging and access channels for callinitiation is well known in the art and is detailed in TIA/EIA InterimStandard IS-95-A, entitled “Mobile Station-Base Station CompatibilityStandard for Dual Mode Wideband Spread Spectrum Cellular System.”

One of the characteristics of CDMA systems is that the same frequency isreused in every cell. Diversity combining is a method by which areceiver receiving signals carrying the same information combines thesignals which are propagated through different paths to provide animproved estimate of a transmitted signal. A receiver design for takingadvantage of the diversity signals carrying the same information buttraveling through different propagation paths or transmitted bydifferent transmitters is described in U.S. Pat. No. 5,109,390 entitled“Diversity Receiver in a CDMA Cellular Telephone System,” assigned tothe assignee of the present invention and incorporated by referenceherein.

Soft hand off is a method by which a mobile station moving from one cellinto another receives information from the base stations serving the twoor more cells at the boundary area as long as the mobile station islocated near the boundary. The signals which are sent by the basestations are combined in the receiver of the mobile station by thediversity combining method mentioned above. A method and system forproviding soft hand-off in a CDMA communication system, where aplurality of base stations are in communication with a mobile station ator near cell boundaries is disclosed in U.S. Pat. No. 5,101,501 entitled“Method and System for Providing a Soft Handoff in Communications in aCDMA Cellular Telephone System,” and U.S. Pat. No. 5,267,261 entitled“Mobile Station Assisted Soft Handoff in a CDMA Cellular CommunicationsSystem,” both of which are assigned to the assignee of the presentinvention and incorporated by reference herein. Hard handoff is, incontrast to soft handoff, where a mobile station passing from one cellto another is dropped by the cell being exited prior to being picked upby the cell being entered.

The use of the same frequency in every cell and the use of soft handoffresult in high CDMA system capacity. The reuse of the same frequency inneighboring cells causes rather rapid changes in the forward link signalto noise ratio near cell boundaries. This is because the cell beingreceived by the mobile station may fade and the neighboring cell mayincrease in strength (anti-fade).

Generally, when the mobile station receives two cells, the receivedtraffic channel energy per spreading chip to total spectral noisedensity for the signal transmitted by cell 1 is given by equation (1)below: $\begin{matrix}{{\frac{E_{c}}{I_{o}}1} = \frac{\frac{E_{c}}{I_{or}}1}{\frac{I_{oc}}{{\hat{I}}_{{or}\quad 1}} + \frac{{\hat{I}}_{{or}\quad 2}}{{\hat{I}}_{{or}\quad 1}} + 1}} & (1)\end{matrix}$And the received traffic channel energy per spreading chip to totalspectral noise density for the signal transmitted by cell 2 is given byequation (2) below: $\begin{matrix}{{\frac{E_{c}}{I_{o}}2} = \frac{\frac{E_{c}}{I_{or}}2}{\frac{I_{oc}}{{\hat{I}}_{{or}\quad 2}} + \frac{{\hat{I}}_{{or}\quad 1}}{{\hat{I}}_{{or}\quad 2}} + 1}} & (2)\end{matrix}$where in equations (1) and (2)

-   -   I_(OC) is the total thermal noise,    -   E_(c)/I_(or1), E_(c)/I_(or2) are the fraction of traffic channel        power transmitted by        -   cell 1 and        -   cell 2 respectively, and    -   Î_(or1), Î_(or2) are the fraction of traffic channel power        received at the mobile station from cell 1 and cell 2        respectively.        Consider I_(OC) to be small relative to Î_(or1) and Î_(or2).        When cell 1 fades relative to cell 2, Î_(or 1) becomes small        relative to Î_(or2) and the ratio Î_(or2)/Î_(or1) , therefore        becomes large.

Thus, E_(c)/I_(o1) , becomes small. If the mobile station is not in softhandoff this change in signal to noise ratio can cause problems.However, if the mobile station is in soft handoff with the neighboringcell, then the change in signal to noise ratio is not a problem becausethe mobile station is performing diversity combining of the forwardtraffic channels from both cells. While the first path given byE_(c)/I_(o1) , becomes small, the second path given by E_(c)/I_(o2) ,becomes large. Thus, fading by one cell increases the signal to noiseratio from the other cell.

Paging is a method of sending information to a mobile station indicatingthe initiation of mobile terminated service or instructing the mobile toreceive new overhead information. A method for initiating a base stationinitiated call is described in detail in U.S. Pat. No. 5,392,287,entitled “Apparatus and Method for Reducing Power Consumption in aMobile Communications Receiver” and in copending U.S. patent applicationSer. No. 08/206,701, filed Mar. 7, 1994, which is a continuation of U.S.Pat. Ser. No. 5,392,287, both of which are assigned to the assignee ofthe present invention and are incorporated by reference herein. Thepresent invention is equally applicable to mobile station initiatedcalls, the method for which is described in copending U.S. patentapplication Ser. No. 08/219,867, filed Jan. 18, 1996 which is assignedto the assignee of the present invention and is incorporated byreference herein.

In a slotted paging system, a mobile station monitors the paging channelfor a short predetermined interval of time and then does not monitor thepaging channel again until the next predetermined time interval. InIS-95-A this method of periodically monitoring the paging channel iscalled slotted mode and the mobile station may monitor the pagingchannel for 80 ms. every 1.28 seconds. The period between monitoringintervals can be made longer as desired by the user. Before eachpredesignated time slot in which a mobile station can be paged, themobile station wakes up (becomes active) and resynchronizes or improvesits synchronization with the base station. The mobile station thenmonitors for pages or other messages in the slot. After an interval, themobile station can become inactive and not monitor the paging channeluntil just before the next assigned slot.

Prior to the time when a mobile station actively communicates trafficinformation with the mobile communication system and after the time whenthe mobile station achieves timing synchronization with thecommunication system, the mobile station is in the idle state. In theidle state the mobile station can receive messages, receive an incomingcall, initiate a call, initiate registration, or initiate messagetransmission. When in the mobile station idle state, IS-95-A permits themobile station to perform an idle handoff at any time other than theinterval that the mobile station is required to monitor its assignedslot.

However, when the mobile station originates a call or receives a page itenters the system access state to send an origination message or a pageresponse message. While in the system access state an IS-95-A mobilestation does not operate in the slotted mode. This is called non-slottedoperation. Specifically, the mobile station continually monitors thepaging channel until directed by the base station to a different stateor an error condition occurs permitting the mobile station to exit thesystem access state. The exemplary embodiment will be described in thecontext of the origination operation and origination message, but theconcepts directly apply to the mobile terminated call process and pageresponse message. After the mobile station sends the origination messageand receives an acknowledgment, the mobile station waits for a channelassignment message, which indicates upon which channel trafficcommunications from the base station to the mobile station will beconducted.

Upon receipt of the channel assignment message, the mobile station tunesto the allocated traffic channel, receives information on the forwardtraffic channel, and begins to transmit on the reverse traffic channel.The forward traffic channel is the channel upon which information fromthe base station to the mobile station is sent and the reverse trafficchannel is the traffic channel upon which information from the mobilestation to the base station is sent.

The interval between the time that the mobile station sends theorigination message and the time when the mobile station receives thechannel assignment message depends upon the implementation of theindividual infrastructure vendor. It can range from less than one-halfof a second to several seconds. Until the time that the mobile stationreceives the channel assignment message the mobile station is in thesystem access state.

The paging channel typically does not support soft handoff. Thus, theissues of fading previously described occur. These are typicallycounteracted by having the radiated power of the paging channel higherthan the traffic channel. Since one paging channel can handle the callorigination and termination of many traffic channels the loss incapacity by this higher power is minimal. In order to support softhandoff on the paging channel, the system sends the same information onthe paging channel in all cells, thus dramatically reducing the overallcapacity of the paging channel.

While in the idle state, the mobile station is permitted to performhandoffs. Typically, the mobile station performs a handoff whenever thereceived signal level from one cell gets sufficiently above anothercell. This idle handoff is typically done before the mobile stationbegins monitoring the slot. However, there can be cases in which themobile station is unable to choose the correct cell before the slotbegins and the mobile station must continue to monitor the existingcell. While in the system access state the mobile station is notpermitted to perform idle handoffs.

However, when the mobile station is in the system access state there canbe cases in which the change in signal to noise ratio, Ec/Io1, is sorapid that the message error rate becomes

too high and the mobile station cannot correctly receive the signalingmessages sent on the paging channel. As a result the mobile station maynot receive the channel assignment message. This causes the callorigination to be unsuccessful. IS-95-A permits the mobile station toexit the system access state and return to the mobile station idle stateif it does not receive any paging channel messages for one second. Thus,the mobile station does not receive the channel assignment message andthe call origination is unsuccessful.

A similar problem exists when the mobile station is first assigned tothe traffic channel. IS-95-A permits only a single base station to beassigned to the mobile station. If another cell is strong or becomesstronger the mobile station may not be able to receive the forwardtraffic channel successfully. As a result the call may drop. The problemis that the mobile station is assigned to a traffic channel with asingle active set member and is not in soft handoff.

Under IS-95-A in order for the mobile station to enter into soft handoffthe following steps must occur. First, the mobile station must determinethat the pilot of another base station is above a predetermined energythreshold value. Second, the mobile station must send a pilot strengthmeasurement message. Third, the infrastructure must set up the handoffand send the handoff direction message to the mobile station. Dependingupon the circumstances and the implementation, this may take from a fewhundred milliseconds to considerably more than one second.

Thus, although soft handoff is generally supported in IS-95-A systems,soft handoff is not supported when the mobile station is in the systemaccess state. There is therefore a need for a system that permits softhandoff while the mobile station is in the system access state toprovide increased reliability in the system access process and otherbenefits.

SUMMARY

In a mobile communication system having a first communication device forcommunicating with a plurality of second communication devices, a methodfor performing handoff within the mobile communication system includestransmitting from at least one of the second communication devices tothe first communication device a handoff list of second communicationdevices to which the first communication device is permitted to handoff.A channel assignment message is transmitted from each of the secondcommunications devices in the handoff list. The handoff list includesbase stations controlled by a single controller that determines which ofthe second communication devices transmits the channel assignmentmessage.

The base stations on the handoff list have pilot signals and the energylevels of the pilot signals are measured by the first communicationdevice. The measured energy levels are transmitted to the controller andcompared by the controller to provide an energy level list of basestations according to the comparing. The energy level list istransmitted to the first communication device and the handoff isperformed according to the energy level list.

The present invention describes further modifications which can improveoperation on the paging and access channels. The present invention thuspermits handoffs while the mobile station is in the system access state.This permits the mobile station to receive a base station whosesignal-to-noise ratio is high so that the message error rate is low. Itavoids dropped call setups due to an inability to receive the pagingchannel.

Another feature of the present invention is that it permits theinfrastructure to determine which base stations should send the channelassignment message to the mobile station. This assures that the mobilestation is able to handoff to a different base station and have atraffic channel allocated to it on the new base station without delay.

A further feature of the present invention is that it permits theinfrastructure to determine which base stations should be in the activeset of the mobile station before the mobile station is assigned to atraffic channel. The active set is a set of base stations providing thestrongest signals to the mobile station. This permits the infrastructureto determine, before the mobile station is assigned to the trafficchannel, whether there are sufficient resources to place the mobilestation into soft handoff. This is useful because a mobile station nearthe cell boundary may immediately request to be placed into soft handoffafter it is assigned to the traffic channel. Furthermore, this minimizescall drops due to the rapid changes in signal-to-noise ratio mentionedpreviously.

Finally the features presented above provide special utility in thepriority access and channel allocation (PACA) operation which allows forusers to gain access to limited communication resources in accordancewith designated user priorities. While the invention is described interms of a CDMA system, the invention is applicable to any cellular orsatellite communications system.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, objects, and advantages of the present invention willbecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

FIG. 1 is a block diagram of the communication system of the presentinvention including a mobile station communicating with a base stationof a group of base stations;

FIG. 2 shows a layout of cells corresponding to the base stations ofFIG. 1;

FIGS. 3A-D show base station lists transmitted to the mobile station inthe communication system of FIG. 1; and

FIG. 4 shows the pilot Ec/I0 for a mobile station moving between twobase stations.

DETAILED DESCRIPTION

Referring now to FIG. 1, there is shown communication system 14.Communication system 14 includes mobile station 2 having a controlprocessor 18 and base stations 26 a-o. In the exemplary embodiment basestations 26 a correspond to cells 36 a in FIG. 2, respectively. Thus,for example, base station 26 a provides coverage for cell 36 a. Uponpower-up mobile station 2 enters the system determination substate. Asystem determination processor (not shown) selects a system upon whichto perform an acquisition attempt and provides the necessary frequencyinformation to receiver 8. Although not separately shown, the systemdetermination processor can be implemented within control processor 18.Control processor 18 can be implemented in a microprocessor ormicrocontroller operating under program control stored in memory.

In the exemplary embodiment mobile station 2 moves into the pilotacquisition substate in which it attempts to demodulate a pilot signalbased upon the acquisition parameters received in the systemdetermination substate and attempts to acquire a CDMA pilot signal inaccordance with the acquisition parameters. Signals (if present) arereceived at antenna 4 and passed through duplexer 6 to receiver 8.Receiver 8 downconverts, amplifies the received signal, converts theanalog signal to a digital representation, and passes the signal tosearcher 10. Searcher 10 attempts to acquire a pilot signal by testingpseudorandom noise (PN) offsets. A PN offset is tested by demodulatingthe signal in accordance with the PN offset hypothesis and measuring thesignal energy of the demodulated signal. Design and implementation ofsearcher hardware for CDMA acquisition is known in the art and isdescribed in detail in aforementioned U.S. Pat. No. 5,109,390.

When searcher 10 detects a pilot signal with energy above apredetermined threshold value, mobile station 2 enters thesynchronization channel acquisition substate and attempts acquisition ofthe synchronization channel. Typically, the synchronization channel asbroadcast by the base stations includes basic system information such asthe system identification (SID) and the network identification (NID),but most importantly provides timing information to mobile station 2.Mobile station 2 adjusts its timing in accordance with thesynchronization channel information and then enters the mobile stationidle state.

Upon successful acquisition of the synchronization channel, mobilestation 2 begins to monitor the paging channel in accordance with apredetermined paging format. Mobile station 2 demodulates a signal basedon a predetermined Walsh sequence that is reserved for paging channeltransmissions. For example if the acquired pilot signal is transmittedby base station 26 a mobile station 2 monitors the paging channel inaccordance with timing information provided by the synchronizationchannel and using a predetermined Walsh sequence. Base station 26 aintermittently transmits overhead information on the paging channel.

Referring now to FIGS. 3A-D, there are shown base station lists 50, 60,70, 80 transmitted to mobile station 2 for permitting idle hand-offwithin communication system 9 of the present invention. In IS-95-Aneighbor list 50 is provided to mobile station 2 by base stations 26 a-nin the neighbor list message. Neighbor list 50 is also referred toherein as NGHBR_LIST_BASE 50. NGHBR_LIST_BASE 50 is a list of basestations 26 b-k in the geographic vicinity of base station 26 a that canprovide strong signals to mobile station 2 and thus are candidates foridle handoff of mobile station 2.

Base station controller 32 is responsible for providing informationbetween base stations 26 a-o, for selectively providing information froma main telephone switching office (not shown) to base stations 26 a-oand for providing base stations 26 a-o with internally generatedmessages. It should be noted that the present invention is equallyapplicable to the case where some of the base stations in neighbor list50 are not controlled by the same base station controller 32.

If the pilot signal acquired by mobile station 2 is transmitted by basestation 26 a after receiving the overhead information, mobile station 2may register with base station 26 a by transmitting its mobileidentification number to base station 26 a. Mobile station 2 then entersthe idle state and monitors its allocated paging channel in the slottedpaging mode after successful registration with base station 26 a. Ifregistration is not performed, mobile station 2 also enters the idlestate and monitors, in the slotted paging mode, its allocated pagingchannel which is transmitted by base station 26 a.

In slotted paging mode, base station 26 a transmits any paging orsignaling information directed to mobile station 2 at predetermined timeintervals called time slots. In the exemplary embodiment the time slotsand paging channel are determined in accordance with a hashing functionof the mobile identification number which upon registration is known tobase station 26 a and mobile station 2.

In the present invention base station 26 a transmits to mobile station 2handoff list 60 of base stations to which mobile station 2 is permittedto perform idle handoff while in the system access state. Handoff list60 is also referred to herein as LIST_BASE 60. Base stations inLIST_BASE 60 are typically a subset of the base stations inNGHBR_LIST_BASE 50 and typically use the same base station controller32. For example, NGHBR_LIST_BASE 50 may include all base stations 26b-k, but LIST_BASE 60 may include only base stations 26 b, 26 c, 26 g,and 26 h.

When mobile station 2 originates a call message generator 20 generatesan origination message and transmits it on the access channel. Messagegenerator 20 can be implemented in a microprocessor programmed to carryout the functions described. Although illustrated as a separate elementmessage generator 20 can be implemented within control processor 18. Theorigination message is received and demodulated by base station 26 awhich the mobile station is currently monitoring. In response toreceiving the origination message each base station in LIST_BASE 60transmits a channel assignment message indicating a traffic channel uponwhich communications can be conducted. Typically the Walsh channel usedfor communications with a first base station in LIST_BASE 60 is notnecessarily the same Walsh channel used for communications with a secondbase station in LIST_BASE 60. Because a plurality of base stations senda channel assignment message the mobile station 2 can perform an idlehandoff while in the system access state and after sending theorigination message to any base station which is in system access andstill be able to receive the channel assignment message.

In an alternative embodiment mobile station 2 sends the originationmessage to base station 26 a and then waits for an acknowledgment of theorigination message. Until mobile station 2 receives the acknowledgmentmobile station 2 is not permitted to perform a handoff. However, aftermobile station 2 receives the acknowledgment it can perform an idlehandoff to any base station in LIST_BASE 60.

In another alternative embodiment mobile station 2 sends the originationmessage using the procedures described in IS-95-A as described in detailin copending U.S. patent application Ser. No. 08/412,648, entitled“Random Access Channel,” filed Mar. 12, 1994 assigned to assignee of thepresent invention and incorporated by reference herein. If anacknowledgment is not received from base station 26 a within apredetermined timeout period the mobile station increases its transmitpower and attempts to send the message again. If mobile station 2 isunable to receive an acknowledgment from base station 26 a after acertain number of attempts and another base station, for example basestation 26 b, is stronger, mobile station 2 is permitted to perform anidle handoff to base station 26 b and restart the transmission of theorigination message.

In one embodiment each of the base stations in LIST_BASE 60 transmits achannel assignment message only indicating a traffic channel forcommunication with itself. In an alternative embodiment each basestation 26 a-i in LIST_BASE 60 transmits an identical channel assignmentmessage that indicates not only the traffic channel to be used forcommunications with that particular base station but also indicates thetraffic channel to be used for communications with all base stations inLIST_BASE 60. This requires the base stations in LIST_BASE 60 tocommunicate the available traffic channels through base stationcontroller 32. By providing channel assignment messages from a pluralityof base stations, the success rate of the channel assignment process isgreatly enhanced.

The above process permits the infrastructure to set up soft handoff andinclude more than one member of the active set in the channel assignmentmessage. Instead of first communicating with one base station and thenmoving into soft hand off it is thus possible for mobile station 2 tocome up immediately in a soft handoff state and immediately receivetraffic communication from two or more base stations. This speeds up theprocess of getting mobile station 2 into soft handoff, which improvesperformance of communication system 9, and minimizes call drops due to alow forward traffic channel signal to noise ratio.

In one embodiment of this process the base station sets up soft handoffwith all base stations in LIST_BASE 60. In an alternative embodiment ofthis process the base station sets up soft handoff with a subset of basestations in LIST_BASE 60 and sends the information in the channelassignment message necessary for mobile station 2 to enter into softhandoff. This information includes the identities of this subset of basestations. In IS-95-A the pilot PN offset identifies the base station.

The paging messages sent by base stations 26 a-i are received at antenna4 of mobile station 2. The received messages are then provided throughduplexer 6 to receiver 8 where the received signal is down converted andamplified. The down converted messages are provided to demodulators 12a-j which demodulate the received messages. Control processor 18 inaccordance with information from searcher 10 selects the incoming pagingchannel or channels that mobile station 2 demodulates. In one embodimentdemodulators 12 a-j monitor only one base station.

Searcher 10 in cooperation with control processor 18 can determine thatanother base station is better. The demodulators then demodulates thereceived signal from the other base station. Because mobile station 2receives an assignment message from more than one base station mobilestation 2 can perform an idle handoff while in the system access state.In another embodiment mobile station 2 monitors all base stations inLIST_BASE 60 and demodulates signals identified in LIST_BASE 60.

In one preferred embodiment LIST_BASE 60 is not provided separately fromneighbor list 50 in the neighbor list message. Rather, composite list 70including all of the members of neighbor list 50 is provided. Flag 72 isalso included within composite list 70 to indicate which members ofneighbor list 50 are also members of LIST_BASE 60. In the exemplaryembodiment one of the reserved values 72 in the overhead message is usedto indicate which base stations specified in the neighbor list messageare in LIST_BASE 60. In an IS-95-A base station reserved values 72 areprovided in the overhead message to specify which members of neighborlist 50 are in LIST_BASE 60.

In the exemplary embodiment the IS-95-A neighbor list message includesthe pilot PN offsets for base stations in the NGHBR_LIST_BASE 50 and anindication of which base stations in the neighbor list message are inLIST_BASE 60. The pilot PN sequence for the current base station istransmitted to provide a reference to mobile station 2 for identifyingthe other base station PN offsets.

As described above all base stations in LIST_BASE 60 are required tosend channel assignment messages to mobile station 2. While this permitsmobile station 2 to handoff and thus increases the success rate of thechannel assignment process, it requires additional paging channelcapacity for all call setups.

A modification of this procedure that reduces the impact upon the pagingchannel capacity is sending a list of base station pilots which areabove a predetermined power threshold to mobile station 2. This list isLIST_MOBILE 80 which is also referred to as energy list 80. In oneembodiment searcher 10 demodulates pilot signals with a preference forthe PN offsets of the base stations in LIST_BASE 60 followed by the PNoffsets of base stations in NGHBR_LIST_BASE 50 and then in accordancewith the remaining PN offsets. A method for providing an optimizedsearch prioritization is described in the aforementioned U.S. Pat. No.5,267,261.

In the exemplary embodiment searcher 10 demodulates the received signalsaccording to a pilot PN offset and measures the energy of thedemodulated pilot. The energy values are provided to control processor18. Control processor 18 compares the energy of the demodulated signalwith a threshold value and compiles energy list 80. Energy list 80contains a list of PN offsets that are above the energy threshold usedby control processor 18. Energy list 80 can be designated LIST_MOBILE80. Once LIST_MOBILE 80 has been compiled it is transmitted on theaccess channel and received by base station 26 a which mobile station 2is monitoring. In the exemplary embodiment LIST_MOBILE 80 is included inthe origination message.

In another embodiment LIST_MOBILE 80 is received by more than one basestation 26 a-o. LIST_MOBILE 80 is provided to base station controller32. In the preferred embodiment the threshold used by mobile station 2to determine whether to include a base station in LIST_MOBILE 80 is sentas part of the overhead messages by base stations 26 a-o. In thepreferred embodiment the threshold can be the T_ADD value sent in theIS-95-A system parameter message. This T_ADD value is currently used bythe IS-95-A mobile stations to determine whether to send the IS-95-Apilot strength measurement message on the traffic channel to the basestation indicating that the mobile station has detected a pilotexceeding T_ADD.

Referring now to FIG. 4., there is shown a graphical representation ofEc/I0 for the IS-95-A pilot channel broadcast by base stations 26 a,b asmobile station 2 moves away from base station 26 a towards base station26 b. When mobile station 2 is fully within the coverage of base station26 a, as indicated by region 38, the pilot channel of base station 26 bis below the T_ADD level. Similarly, when mobile station 2 is fullywithin the coverage of base station 26 b, as indicated by region 41, thepilot channel of base station 26 a is below the T_ADD level. When mobilestation 2 is in region 38, it does not report in the origination messagebase station 26 b. Similarly, when mobile station 2 is in region 41, itdoes not report, in the origination message, base station 26 a.

When mobile station 2 is in region 39 the pilot Ec/I0 for base station26 b is above T_ADD and the mobile station reports base station 26 b inthe origination message. Similarly, when mobile station 2 is in region40 the pilot Ec/10 for base station 26 a is above T_ADD and the mobilestation 2 reports base station 26 a in the origination message. Thepreferred embodiment uses Ec/I0 as disclosed in IS-95-A for thesemeasures. However, alternative measures of signal strength or signal tonoise ratio which are well known in the art are equally applicable.

In the preferred embodiment mobile station 2 is permitted to perform anidle handoff only to base stations in both LIST_MOBILE and LIST_BASE 60.The set of base stations in both lists can be designated LIST_BOTH. Thishas two advantages. First, the infrastructure need only send the channelassignment message in those base stations identified by the mobilestation as possible candidates for an idle handoff and which the mobilestation is permitted to handoff. This is the set of base stations givenin LIST_BOTH. This significantly reduces the additional messaging thatis required. Second, LIST_MOBILE provides a list of pilots above T_ADDto base station controller 32. This permits the infrastructure toidentify which base stations should be a member of the active set of themobile station. Thus, for base station controller 32 to set up softhandoff when the mobile station is assigned to a traffic channel it canset up soft handoff only with the base stations in LIST_MOBILE.

In an alternative embodiment mobile station 2 sends the base stations inLIST_BOTH to the base station in its origination message. This reducesthe amount of information that must be sent from mobile station 2.Furthermore, it permits the infrastructure to set up soft handoff andinclude more than one member of the active set in the channel assignmentmessage. The channel assignment message would include the pilot PNoffsets of base stations which are in the active set. It is possible forthe mobile station 2 to come up immediately in a soft handoff state andimmediately receive traffic communication from two or more basestations, instead of first communicating with one base station and thenmoving into soft hand-off which may not be possible due to capacity orother limitations.

For example, if mobile station 2 is at location 37 in cell 36 a near theboundary with cell 36 b LIST_MOBILE identifies the pilot PN offsets ofbase station 26 b. The channel assignment message transmitted by basestations 26 a,b identifies a traffic channel for use by mobile station 2for dedicated communications between base stations 26 a,b and mobilestation 2. At least one demodulator 12 a-j is tuned to receive trafficchannel information from base stations 26 a and another demodulator 12a-j is tuned to receive traffic channel information from base station 26b. A plurality of demodulators 12 a-j begin demodulating the trafficchannel signals transmitted by base stations 26 a,b. The demodulatedsignals are applied to diversity combine 34 which combines the receivedsignals to provide an improved estimate of the transmitted data.

In practicing the method of the present invention mobile station 2 doesnot perform an idle handoff until it receives an acknowledgment for atransmitted message or a timeout indicating that the period foracknowledgment has expired. This permits mobile station 2 to receive theacknowledgment of its access channel probes. It also permits the basestation 26 a to which the mobile station is sending its access channelprobes to generate the acknowledgment rather than the acknowledgmentbeing generated by base station controller 32. This reduces delay, andmakes the call setup process faster. Furthermore, if mobile station 2 isin the system access state and performs an idle handoff after theacknowledgment timeout expires, mobile station 2 must restart the accesschannel probe transmission procedures. This would be the same as ifmobile station 2 transmitted a new origination message.

In an alternative embodiment, mobile station 2 performs the idle handoffto base stations in LIST_BASE 60 before receiving the acknowledgment.Thus, all base stations in LIST_BASE 60 must send the acknowledgment andbase station controller 32 must help generate the acknowledgments. In amodification of this alternative embodiment mobile station 2 may performthe idle handoff to base stations in LIST_MOBILE before receiving theacknowledgment. Similarly, all base stations in LIST_MOBILE must sendthe acknowledgment and thus base station controller 32 must be involvedin generating the acknowledgments.

In a preferred embodiment the present invention provides for thecontingency that the channel assignment message was transmitted by basestation 26 a but not received by mobile station 2. Base station 26 a mayhave received the origination message from mobile station 2 but mobilestation 2 may not have received the channel assignment messageacknowledging the receipt of the origination message from base station26 a. Even absent receipt of the acknowledgment message mobile station 2may perform the idle handoff to, for example, base station 26 b.

Base station 26 b send the channel assignment message to mobile station2, while mobile station 2 retransmits the origination message. In theexemplary embodiment when an acknowledgment message is sent anindication of which message is being acknowledged accompanies it. Mobilestation 2 ignores the channel assignment message unless the indicatorcorresponds to the most recently sent origination message. The presentinvention includes several ways to correct this problem. One solution isfor base station 26 b to use the same acknowledgment indicator that isin the origination message received by base station 26 a. This can bedone by passing the acknowledgment indicator values from base station 26a to base station 26 b through base station controller 32. In analternative embodiment mobile station 2 can stop transmitting an accessprobe if it receives the channel assignment message and tune to thechannel specified by the channel assignment message.

In an improved embodiment the paging channel configuration of all basestations to which mobile station 2 is allowed to handoff (base stationsin LIST_BASE 60) are the same. Base stations not supporting thesecapabilities would not be included in LIST_BASE 60.

This method can also be used to support PACA. The PACA feature is wellknown in the art and is described in detail in “TIA/EIA/IS-53-A CellularFeatures Description.” When PACA is invoked mobile station 2 is givenpriority over other mobile stations in obtaining a traffic channel whena traffic channel is not available. Specifically, mobile station 2 sendsan origination message containing the PACA feature code and the dialednumber. If a traffic channel is immediately available, mobile station 2is assigned to the traffic channel. If a traffic channel is notimmediately available and mobile station 2 is authorized to use PACA,the base station monitoring mobile station 2 places the request ofmobile station 2 into a PACA queue. For example, base station 26 a canplace the request in the queue. Alternatively, the PACA queue can bemanaged by base station controller 32. The position in the queue dependsupon the priority of the PACA request and the age of the request. When atraffic channel becomes available, the request at the head of the PACAqueue is assigned to the traffic channel.

When the request of mobile station 2 is in a PACA queue mobile station 2can receive periodic messages informing the user of mobile station 2 ofthe queue status. One issue with PACA is that the infrastructure needsto know the cell which mobile station 2 is currently using in order todetermine whether the channel is free. With most systems this requiresmobile station 2 to register or resend the origination message everytime mobile station 2 performs an idle handoff. Due to the abruptness oftransitions between CDMA base stations mobile station 2 may register orresend the origination message several times while crossing the boundarybetween base stations. A second consideration with CDMA is that mobilestation 2 may go into soft handoff soon after it is assigned to atraffic channel. Unless resources are available in multiple basestations to support the call the assignment may not be successful.

With the modified origination message described above mobile station 2indicates other base stations that should be in the active set of themobile, the set of base stations from which mobile station 2 hasdetected strong pilot signals. In one embodiment mobile station 2 sendsLIST_MOBILE and the base station determines LIST_BOTH. In anotherembodiment, mobile station 2 sends LIST_BOTH. This permits theinfrastructure to determine whether resources are free in all basestations needed for the PACA call. In order to reduce the rate ofsending the origination message, the base stations in LIST_BOTH arethose which the mobile station can move without having to resend theorigination message. When this feature is invoked the infrastructureneeds to send the queue status information in all base stations in LISTBOTH. If mobile station 2 moves out of the coverage of the base stationsin LIST_BOTH, mobile station 2 resends the origination message.

The previous description of the preferred embodiments is provided toenable any person skilled in the art to make or use the presentinvention. The various modifications to these embodiments will bereadily apparent to those skilled in the art, and the generic principlesdefined herein may be applied to other embodiments without the use ofthe inventive faculty. Thus, the present invention is not intended to belimited to the embodiments shown herein but is to be accorded the widestscope consistent with the principles and novel features disclosedherein.

1. An apparatus for performing handoff within a mobile communicationsystem, the mobile communication system having at least one firstcommunication device for communicating with a plurality of secondcommunication devices (SCD), comprising: means for transmitting from atleast one of the SCDs to the first communication device a handoff listof SCDs to which the first communication device is permitted to handoffwhen in a system access state; and means for transmitting a channelassignment message to the at least one first communication device fromeach of the SCDs in the handoff list.
 2. The apparatus in accordancewith claim 1, wherein the channel assignment message transmitted fromeach SCD in the handoff list includes a channel assignment message to beused only for communicating with the respective transmitting SCD.
 3. Theapparatus in accordance with claim 1, further comprising means fortransmitting to the first communication device a neighbor list of SCDsthat are located in the geographic vicinity of the first communicationdevice.
 4. The apparatus in accordance with claim 1, further comprisingmeans for performing a handoff of the first communication device fromone of the SCDs to another one of the SCDs.
 5. A communication devicefor communicating with a plurality of second communication devices(SCD), comprising: a controller; and a receiver for receiving from atleast one of the SCDs a handoff list of SCDs to which the communicationdevice is permitted to handoff when in a system access state, andreceiving a channel assignment message from each of the SCDs in thehandoff list.
 6. The device in accordance with claim 5, wherein thechannel assignment message transmitted from each SCD in the handoff listincludes a channel assignment message to be used only for communicatingwith the respective transmitting SCD.
 7. The device in accordance withclaim 5, wherein the receiver further receives a neighbor list of SCDsthat are located in the geographic vicinity of the first communicationdevice.
 8. The device in accordance with claim 5, wherein the controllerperforms a handoff of the communication device from one of the SCDs toanother one of the SCDs.
 9. A apparatus for performing handoff within amobile communication system, the mobile communication system having atleast one first communication device for communicating with a pluralityof second communication devices (SCD), comprising: means fortransmitting from at least one of the SCDs to the first communicationdevice a handoff list to which the first communication device ispermitted to handoff when in a system access state, wherein the handofflist comprises SCDs controlled by a single controller not located in theat least one first communication device; and means for transmitting achannel assignment message to the at least one first communicationdevice from each of the SCDs in the handoff list.
 10. The apparatus inaccordance with claim 9, wherein the single controller determines theSCDs used to transmit a channel assignment message.
 11. The apparatus inaccordance with claim 9, wherein the SCDs on the handoff list transmitpilot signals, the apparatus further comprising: means for measuring theenergy levels of the pilot signals at the first communication device;means for transmitting the measured energy levels to the singlecontroller; means for comparing the transmitted energy levels; and meansfor providing an energy level list of SCDs according to the comparison.12. The apparatus in accordance with claim 11, further comprising meansfor transmitting the energy level list to the first communicationdevice.
 13. The apparatus in accordance with claim 12, furthercomprising means for performing a handoff according to the energy levellist.
 14. The apparatus in accordance with claim 13, comprising meansfor performing handoff only with SCDs contained in both the handoff listand the energy level list.
 15. The apparatus in accordance with claim11, further comprising means for transmitting to the first communicationdevice a neighbor list of SCDs that are located in the geographicvicinity of the first communication device, wherein the handoff ispreceded by the step of transmitting from the first communication devicean origination message to at least one SCD.
 16. The apparatus inaccordance with claim 15, wherein the handoff list and the neighbor listare transmitted together.
 17. The apparatus in accordance with claim 15,wherein the neighbor list comprises SCDs controlled by a plurality ofcontrollers.
 18. The apparatus in accordance with claim 15, wherein theSCDs on the handoff list comprise a subset of the SCDs on the neighborlist.
 19. The apparatus in accordance with claim 9, further comprisingmeans for performing a handoff of the first communication device fromone of the SCDs to another one of the SCDs when the first communicationdevice is in an idle mode.
 20. The apparatus in accordance with claim 9,further comprising means for synchronizing by the first communicationdevice with at least one of the SCDs.